Acrylic acid is a valuable industrial commodity and has a variety of uses. Polymers made from acrylic acid are used for the manufacture of adhesives, binders, coatings, paints, polishes and superabsorbent polymers, the latter in turn being used in disposable absorbent articles including diapers and hygienic products, for example.
Currently acrylic acid is made from petroleum source materials. For example, acrylic acid has long been prepared by the catalytic oxidation of propylene. In recent years, however, with an increasing awareness of the need to develop renewable source-based processes for the manufacture of acrylic acid and other conventional petrochemicals, significant amounts of research have been devoted to the identification and development of processes for making acrylic acid from renewable resources.
A number of references thus describe methods for converting glycerol to acrylic acid and/or acrylates, commonly using glycerol such as that produced in the making of biodiesel (fatty acid methyl esters) from plant oils, see, e.g., U.S. Pat. No. 7,396,962 to DuBois et al. and references cited therein.
Of more immediate relevance to the process of the present invention, a number of efforts have likewise been made to develop processes for making acrylic acid from carbohydrate and/or carbohydrate-derived feedstocks. One feedstock that can be derived from carbohydrates and that has been closely evaluated is 3-hydroxypropionic acid, or 3-HPA. U.S. Pat. No. 2,859,240 to Holmen (1958) indicates that the dehydration of 3-HPA is a “comparatively simple and economical process”, but concludes that “the starting material is neither low in cost or readily available in quantity” (col. 1, lines 55-58) Essentially the same assessment is offered 45 years later, wherein in Kumar et al., “Recent advances in biological production of 3-hydroxypropionic acid”, Biotechnology Advances, vol. 31, pp. 945-961 (2013), the authors conclude despite “significant progress” in the preceding decade toward “commercial production . . . in the near future” that “many important issues still remain and require more extensive investigations.”
Another feedstock that can be derived from carbohydrates and that has been the subject of considerable research as well is lactic acid. In the same 1958 Holmen patent, for example, lactic acid is indicated as having been recognized for some time as preferable to 3-HPA as a prospective feedstock due to its ready availability (referencing a 1950 review of efforts to that time to develop processes for converting lactic acid and the lower alkyl esters of lactic acid to acrylic acid and the corresponding lower alkyl esters of acrylic acid). A commercially viable process yet remains elusive as well for the conversion of lactic acid to acrylic acid, as evidenced by a number of ongoing applications for patent that have recently been filed.
WO 2012/033845 to Ozmeral et al, WO 2012/156921 to Dongare et al. and WO 2013/155245 to Lingoes et al. are representative of these ongoing efforts to develop a commercially viable process for converting lactic acid (and/or lactate esters) to acrylic acid (and/or the corresponding acrylate esters), and each in turn reviews a fairly substantial body of additional published art detailing prior work toward the same objective.
In WO 2012/033845, a fermentation broth containing ammonium lactate is described as processed according to one of four pathways to produce acrylic acid esters. In a first pathway, lactic acid is first purified from the fermentation broth. The highly purified lactic acid is then subjected to a vapor phase dehydration reaction at elevated temperatures and in the presence of an appropriate catalyst to produce acrylic acid, which in turn is esterified in the presence of an esterification catalyst to provide the acrylate esters. In a second pathway, lactic acid in the fermentation broth is dehydrated “without much purification”, followed by an esterification to produce acrylic acid esters. In the third pathway, ammonium lactate in the fermentation broth is subjected to simultaneous dehydration and esterification reactions to produce an acrylic acid ester product, while in the fourth pathway, ammonium lactate in the fermentation broth without much purification is subjected first to an esterification reaction to produce a lactic acid ester, and then this lactic acid ester is dehydrated to provide an acrylic acid ester product. In a “most preferred” embodiment according to this fourth pathway, a fermentation broth containing ammonium lactate is concentrated by evaporation of water and subjected to esterification with a C1-C10 alkyl alcohol, preferably in the absence of any exogenous esterification catalyst. Ammonia released during the concentration process is captured for recycling to the lactic acid fermentation, along with further ammonia released during the esterification reaction. The lactic acid ester obtained in the first stage is then dehydrated to produce a corresponding acrylic acid ester.
In WO 2012/156921 to Dongare et al., a catalyst with improved selectivity to acrylic acid from lactic acid and reduced production of acetaldehyde and other products is offered for use in the dehydration of lactic acid to acrylic acid, comprising a calcium phosphate in a calcium to phosphate ratio of from 1.5 to 1.9 as optionally modified with 5 weight percent of sodium. The process is described as involving preheating the catalyst in a fixed-bed reactor at a temperature of 370 to 380 degrees Celsius for from 20 to 40 minutes under highly pure nitrogen, then passing 50-80 wt. pct preheated vapors of a lactic acid solution through a quartz fixed catalyst bed reactor by means of a nitrogen carrier gas. Reported lactic acid conversion under these conditions was 100 percent, with 60 to 80 percent selectivity for acrylic acid and 15-35 percent selectivity for acetaldehyde.
In WO 2013/155245 to Lingoes et al., reference is made initially to research by a number of parties of a similar character to that reported in Dongare et al., which research confirmed that phosphate and nitrate salts may desirably change the surface acidity of acidic catalysts to inhibit the decarbonylation/decarboxylation of lactic acid to acetaldehyde in particular.
Lingoes et al. contend that even with a reduced selectivity to acetaldehyde, nevertheless even the reduced amounts are problematic, as byproducts can be deposited on the catalyst and result in fouling and in premature and rapid deactivation of the catalyst. Further, once deposited, the byproducts can catalyze other undesired reactions, for example, polymerization reactions (para. 0005).
As well, apart from the difficulties caused by being deposited on the catalyst in question, Lingoes et al. point out the difficulties even very small amounts of byproducts such as acetaldehyde, propanoic acid, carbon monoxide, carbon dioxide, 2-3-pentanedione and lactic acid oligomers can cause in processing acrylic acid from the then-known lactic to acrylic processes to make superabsorbent polymers, such that a significant body of literature existed around removal of these impurities from the acrylic acid.
Lingoes et al. reference U.S. Pat. No. 6,541,665 and U.S. Published Pat. Appl. 2011/0257355 as exemplars of this body of literature. In U.S. Pat. No. 6,541,665, a 5-stage crystallization (containing two purification stages and three stripping stages) was effective to obtain 99.94% acrylic acid containing 2600 parts per million by weight of acetic acid and 358 ppm of propanoic acid, among other species. In U.S. 2011/0257355, a method is described of removing propanoic acid in a single pass crystallization from a crude reaction mixture derived from glycerol dehydration/oxidation to obtain 99% acrylic acid. According to Lingoes et al, prior to their improved catalyst and process, the prior art methods for converting lactic acid to acrylic acid produced amounts of byproducts that were too high (“far too high”) to even utilize such purification methods.